62 NATURAL HISTORY OF WASPS. 
seem to be larger or smaller, according as the scales 
of the abdomen are drawn out or retracted, but the 
frame does not alter; and indeed the only permanent 
change of the perfect insect is a constant shrinking 
up and diminution of size with advancing days. The 
wasp is no exception to the ordinary rule of insects; 
the skeleton has no means of growth any more than 
of repair. The seeming exceptions to the rule, where 
the form of the perfect insect is maintained through- 
out life, and is put on before the great final change 
is accomplished, are not found among flying insects. 
Little bugs and little cockroaches are early éndued 
with the form which they are to bear for life, and 
within this frame they carry the materials and organs 
by which they are to be developed, by successive 
stages, into more and more perfect insects. But flies, 
butterflies, and wasps do not take their aérial form 
till they are past change, and can rise unburthened 
into their proper element. The variations in the size 
of the Hymenoptera are due to original development 
of the pupa, not to later growth of the perfect insect. 
The integument of the wasp, as of other insects, 
is composed of chitin, a substance resembling horn 
in some of its mechanical properties, but totally 
unlike it in structure and chemical composition. 
Chemically examined, chitin* is found to be insoluble 
in a solution of caustic potash; it does not swell up 
and melt before the flame of the blow-pipe, but burns 
away into a white ash composed of phosphate of lime 
and iron, with carbonate of potash. And the cover- 
* ‘Cyclopedia of Anatomy and Physiology,’ Vol. II, p. 882. See 
also Lehmann, ‘ Handbuch der Physiologischen Chemie,’ Leipzig, 
1859, p. 192, on the Chemical Affinities of Chitin. 
